Second surface laser ablation

ABSTRACT

A method of removing material from an opposite side of workpiece includes directing a laser beam at a first side of the workpiece to remove the material from an opposite second side of the workpiece.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/059,351, filed on Oct. 3, 2014, the entire disclosureof which is incorporated herein by reference for any and all purposes.

TECHNICAL FIELD

The present disclosure generally relates to laser ablation processes andproducts produced thereby.

SUMMARY

In a first aspect, a method is provided comprising: providing aworkpiece comprising a substrate, a coating layer disposed over a firstsurface of the substrate, and a mask disposed over a second surface ofthe substrate; and removing a portion of the coating layer from theworkpiece by directing a laser beam at the workpiece such that the laserbeam passes through the substrate from the second surface to the firstsurface before the laser beam impinges on the coating layer. The firstsurface of the substrate and the second surface of the substrate areopposite surfaces of the substrate, and the mask selectively preventsremoval of the coating layer from portions of the workpiece. The methodmay further comprise disposing the mask over the second surface of thesubstrate. Disposing the mask may comprise a photolithography process ora printing process. Alternatively, disposing the mask may compriseplacing a pre-formed mask over the second surface of the substrate.Removing the portion of the coating layer may produce portions of theworkpiece from which the coating layer has been removed that have acharacteristic dimension that is smaller than a laser spot produced bythe laser beam. An edge of the coating layer adjacent the portion of theworkpiece from which the coating layer has been removed may have acharacteristic length, L, that is a length of the portion of the edge ofthe coating layer in which a thickness of the coating layer tapers froma nominal coating layer thickness, t, to zero thickness, and L is 100 μmor less. Alternatively, L may be 50 μm or less, such as 200 nm or less.An edge of the coating layer adjacent the portion of the workpiece fromwhich the coating layer has been removed may have a uniform shape. Anedge of the coating layer adjacent the portion of the workpiece fromwhich the coating layer has been removed may have a scalloped profile. Adepth of the scallops may be between 1% and 20% of a diameter of thelaser beam. A depth of the scallops may be less than 100 μm. The laserbeam may be produced by a laser with a pulse duration in a range of 0.5to 500 picoseconds. The workpiece may also comprise an electricallyconductive layer disposed between the substrate and the coating layer,and the electrically conductive layer is not removed by the laser beam.The electrically conductive layer may comprise a transparent conductiveoxide, such as indium tin oxide. The coating layer may comprise ametallic material, such as chromium. The coating layer may comprisemultiple layers. An edge of the coating layer adjacent the portion ofthe workpiece from which the coating layer has been removed may form anaverage angle with the first substrate surface in a range of 30° to 90°.The portion of the workpiece from which the coating layer is removed mayexhibit a transmission haze of 0.25% or less, such as 0.05% or less.

In a second aspect, a method is provided comprising: providing aworkpiece comprising a substrate and a coating layer disposed over afirst surface of the substrate; and removing a portion of the coatinglayer from the workpiece by directing a laser beam at the workpiece suchthat the laser beam passes through the substrate before the laser beamimpinges on the coating layer. The laser beam is produced by a laserwith a pulse duration in a range of 0.5 to 500 femtoseconds. Theworkpiece may also comprise a mask disposed over a second surface of thesubstrate, with the first surface of the substrate and the secondsurface of the substrate being opposite surfaces of the substrate. Themask may selectively prevent removal of the coating layer from portionsof the workpiece.

In a third aspect, a product is provided comprising: a substrate; acoating layer disposed over a first surface of the substrate; and aportion of the first surface of the substrate that is substantially freeof the coating layer. An edge of the coating layer adjacent the portionof the first surface of the substrate that is substantially free of thecoating layer forms an average angle with the first substrate surface ina range of 30° to 120°, the edge of the coating layer adjacent theportion of the first surface of the substrate that is substantially freeof the coating layer has a characteristic length, L, that is a length ofthe portion of the edge of the coating layer in which a thickness of thecoating layer tapers from a nominal coating layer thickness, t, to zerothickness, and L is 100 μm or less. The characteristic length L may be50 μm or less, such as 200 nm or less. The product may also comprise anelectrically conductive layer disposed between the substrate and thecoating layer and over the portion of the first surface of the substratethat is substantially free of the coating layer. The coating layer maycomprise a metallic material. The portion of the first surface of thesubstrate that is free of the coating layer may exhibit a transmissionhaze of 0.25% or less, such as 0.05% or less. The edge of the coatinglayer adjacent the portion of the first surface of the substrate that issubstantially free of the coating layer may have a scalloped profile. Adepth of the scallops may be less than 100 μm, such as less than 50 μm.The product may also comprise: a first electrode layer; a secondelectrode layer; and an electrochromic layer. The electrochromic layeris disposed between the first electrode layer and the second electrodelayer, and the first electrode layer is disposed over the portion of thefirst surface of the substrate that is free of the coating layer.

In a fourth aspect, a vehicle rearview mirror assembly is provided thatcomprises the product of the third aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments will hereinafter be described in conjunctionwith the appended drawings, wherein like designations denote likeelements.

FIG. 1 is a side cross-sectional view of an embodiment of a secondsurface laser ablation process.

FIG. 2 is a top view of the process of FIG. 1.

FIG. 3(a) is a cross-sectional view of a processed edge of a laserablated coating layer.

FIG. 3(b) is a cross-sectional view of an edge of a coating formed bymasking in a material addition process.

FIG. 4(a) is a top view of the processed edge of the laser ablatedcoating layer.

FIG. 4(b) is a top view of the edge of the coating formed by masking inthe material addition process.

FIG. 5 is a top view of the process of FIG. 1 illustrating analternative process path.

FIG. 6 is a component formed from ablated workpiece of FIG. 5.

FIG. 7 is a side cross-sectional view of an embodiment of the secondsurface laser ablation process, where the workpiece includes anadditional material layer.

FIG. 8 is a side cross-sectional view of an embodiment of a masked laserablation process.

FIG. 9(a) is a photomicrograph of a laser ablated edge formed by apicosecond laser.

FIG. 9(b) is a photomicrograph of a laser ablated edge formed by ananosecond laser.

DETAILED DESCRIPTION

Various embodiments are described hereinafter. It should be noted thatthe specific embodiments are not intended as an exhaustive descriptionor as a limitation to the broader aspects discussed herein. One aspectdescribed in conjunction with a particular embodiment is not necessarilylimited to that embodiment and can be practiced with any otherembodiment(s).

As used herein, “about” will be understood by persons of ordinary skillin the art and will vary to some extent depending upon the context inwhich it is used. If there are uses of the term which are not clear topersons of ordinary skill in the art, given the context in which it isused, “about” will mean up to plus or minus 10% of the particular term.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the elements (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the embodiments and does not pose alimitation on the scope of the claims unless otherwise stated. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential.

A laser ablation process generally includes selective removal ofmaterial at a surface of a workpiece by directing a laser beam at theworkpiece. The laser beam is configured to deliver a controlled amountof energy at a laser spot defined where the beam impinges the desiredsurface. This controlled amount of energy is selected to liquefy,vaporize, or otherwise rapidly expand the surface material at the laserspot to cause it to separate from the workpiece for removal. Laserablation can be used to remove at least a portion of one or morecoatings from a coated substrate, for example, or to otherwise reshapethe workpiece surface.

The present application discloses a method of laser ablation of acoating layer from a first surface of a substrate by passing a laserbeam through the substrate before the laser beam impinges on the coatingmaterial. The method includes providing a workpiece comprising asubstrate, a coating layer disposed over a first surface of thesubstrate, and a mask disposed over a second surface of the substrate;and removing a portion of the coating layer from the workpiece bydirecting a laser beam at the workpiece such that the laser beam passesthrough the substrate from the second surface to the first surfacebefore the laser beam impinges on the coating layer. The first surfaceof the substrate and the second surface of the substrate are oppositesurfaces of the substrate, and the mask selectively prevents removal ofthe coating layer from portions of the workpiece.

According to another embodiment, a method includes providing a workpiececomprising a substrate and a coating layer disposed over a first surfaceof the substrate; and removing a portion of the coating layer from theworkpiece by directing a laser beam at the workpiece such that the laserbeam passes through the substrate before the laser beam impinges on thecoating layer. The laser beam is produced by a laser with a pulseduration in a range of 0.5 to 500 femtoseconds.

According to another embodiment, a product includes a substrate having acoating layer disposed over a first surface of the substrate, and aportion of the first surface of the substrate is substantially free ofthe coating layer. An edge of the coating layer adjacent the portion ofthe first surface of the substrate that is substantially free of thecoating layer forms an average angle with the first substrate surface ina range of 30° to 120°. The edge of the coating layer adjacent theportion of the first surface of the substrate that is substantially freeof the coating layer has a characteristic length, L, that is a length ofthe portion of the edge of the coating layer in which a thickness of thecoating layer tapers from a nominal coating layer thickness, t, to zerothickness, and L is 100 μm or less. A vehicle rear view mirror assemblymay include the product.

Turning to the figures to illustrate various embodiments, FIG. 1 is aside cross-sectional view of an example of a laser ablation process asperformed on a workpiece 10. The workpiece 10 is a coated substrate,including a substrate 12 and a coating layer 14. The illustrated processis a second surface ablation process in which the coating layer 14 islocated at a second side 16 of the workpiece 10 opposite a first orimpingement side 18 of the workpiece. A laser beam 100 is provided by alaser source 102 and propagates toward the workpiece. In this example,the laser beam 100 is configured with a focal plane at or near a secondsurface 20 of the substrate 12 and generally parallel with the x-yreference plane to define a laser spot 104 at the second surface. Inother examples, the focal plane can be spaced from the second surface 20by an amount greater than 0 mm up to about 50 mm. The substrate 12 is atleast partially transparent to the particular wavelength of laser lightof the laser beam 100 so that the beam passes through the thickness ofthe substrate to the second surface 20, where the material of thecoating layer 14 absorbs at least some of the energy of the laser beamand is thereby separated from the substrate.

In the example of FIG. 1, the removed coating layer material 22 isillustrated in the form of solid particles. The workpiece 10 can beoriented as shown so that gravity causes the removed material 22 to fallaway from the workpiece 10. Optionally, a vacuum source 106 is providedto help guide the removed material 22 away from the workpiece 10. Theremoved material 22 may be in vapor or liquid form when initiallyseparated from the substrate 12. The illustrated arrangement is usefulto prevent the removed material 22 from being redeposited on theworkpiece 10, which can be problematic with some first surface ablationprocesses. The material may also be removed via a spallation process.

In order to remove material from an area of the workpiece 10 that islarger than the laser spot 104, the laser beam 100 and/or the workpiece10 may be moved relative to each other to remove material at a pluralityof adjacent and/or overlapping laser spot locations. For instance, afterthe desired amount of material is removed at a first laser spotlocation, the workpiece 10 and/or laser beam 100 may move to define asecond laser spot location for further removal of material. Continuedmovement to multiple adjacent or overlapping laser spot locations withcorresponding material removal at each location defines an ablated area24 of the workpiece 10 from which material has been removed, as shown ina top view of the process in FIG. 2, where an intended ablation area 26is shown in phantom. In FIGS. 1 and 2, the laser beam 100 is moving inan instant process direction A with respect to the workpiece 10. One orboth of the laser beam 100 or the workpiece 10 may be moved to achievethis relative movement. In one example, the laser beam 100 moves orscans back and forth in the positive and negative x-direction within theintended ablation area 26, and the laser beam and/or the workpiece 10 isindexed in the y-direction each time the laser beam reaches an edge 28of the intended ablation area until the coating layer 14 is removedwithin the entire intended area.

A high-frequency pulsed laser may be used in conjunction with workpiece10 and/or laser beam 100 movement at a particular rate in the processdirection to determine the spacing between adjacent laser spotlocations. In a non-limiting example, a laser beam operating with apulse frequency of 400 kHz with a rate of movement with respect to theworkpiece of 20 m/s in the process direction will result in laser spotlocations every 50 μm in the process direction. Laser spot locationsthus overlap when the cross-sectional dimension of the laser beam 100,measured in the process direction, is greater than the spacing betweenadjacent laser spot locations. A single pulse or a pulse burst may bedelivered at each laser spot location, where the pulse durations aregenerally one or more orders of magnitude less than the time betweenpulses. Spacing among laser spot locations may be selected so thatadjacent spot locations at least partially overlap to ensure materialremoval between adjacent locations, particularly with non-rectangularbeam cross-sections.

The illustrated process is useful as an alternative to material additionprocesses to form a product with coated and uncoated portions. Materialaddition processes (e.g., painting, plating, vapor deposition,sputtering, etc.) typically require the use of some form of masking tohelp define a boundary between the coated and uncoated portions bypreventing the coating material from being deposited at the desireduncoated portion. In such processes, a physical mask may be placedbetween a coating material source and the substrate to physically blockthe coating material at the desired uncoated portion(s) of thesubstrate, or a resist coating material may first be coated onto thedesired uncoated portion (while masking the desired coated portion) andsubsequently removed after the coating material is deposited over thesubstrate, including over the resist coating layer.

In the above-described laser ablation process, the workpiece 10 can bepresented with the coating layer 14 at both of the desired coated anduncoated portions (e.g. an entire substrate surface), and the coatinglayer can be selectively removed to form the uncoated portion (i.e., theintended ablation area 26). The laser ablation process can reduce oreliminate the need for the additional tooling and process steps that arerequired in material addition processes where it is desired to coat onlya portion of the substrate. The laser ablation process is also moreflexible, as the size and/or shape of the intended ablation area 26(i.e., the uncoated portion of the product) can be changed through arelatively simple reprogramming of the laser system without the need toclean or produce new physical components such as masks.

In addition, newly formed edges of the coating layer 14 may be betterdefined than corresponding edges of a coating deposited in a materialaddition process. This phenomenon is illustrated schematically in FIGS.3 and 4. FIG. 3(a) is a cross-sectional view of a processed edge 30 ofthe coating layer of FIG. 2, and FIG. 3(b) is a cross-sectional view ofan edge 30′ of a coating layer applied in a material addition processwith masking. As shown in FIG. 3(a), the processed edge 30 can be formedat an angle θ with the underlying surface 20 that is substantiallyperpendicular or near perpendicular. The coating layer produced bymasking and coating deposition shown in FIG. 3(b) has an edge 30′ thatgradually tapers from the full thickness of the coating layer over amuch greater characteristic length L and forms a much lower averageangle θ with the substrate, due in part to shadowing effects from themask. The laser ablation process can produce an angle θ between the edge30 and the substrate surface 20 in a range from 30 degrees to 90degrees, or from 30 degrees to 120 degrees. In some applications, anangle θ on the higher end of this range may be preferred, such as arange from 70 degrees to 90 degrees.

The characteristic length L of the taper from full thickness to zerothickness can be related to the laser spot size and/or the coatingthickness in the laser ablation process. In some embodiments, thecharacteristic length L is less than or equal to one half of thediameter or width of the laser spot. Thus, for a 200 μm diameter laserspot, the characteristic length L may be 100 μm or less. In some cases,the characteristic length L is less than or equal to one quarter of thediameter or width of the laser spot—i.e., 50 μm or less with a 200 μmlaser spot. The characteristic length L may be less than or equal totwice the nominal thickness of the coating layer 14, such that a 100 nmcoating layer may have a processed edge 30 that tapers from 100 nm tozero over a length of 200 nm or less. In other examples, thecharacteristic length L may be up to 10 times the nominal thickness ofthe coating layer. In other embodiments, the characteristic length L isless than or equal to the nominal thickness of the coating layer 14, oronly a fraction of the nominal thickness of the coating layer, such asfrom 0.01 to 0.99 times the nominal thickness of the coating layer. Inembodiments where θ is near 90 degrees, for example, the characteristiclength L may be in a range from about 0.01 to about 0.10 times thenominal thickness of the coating layer. In other embodiments, thecharacteristic length L may be in a range from about 0.01 to about 1.0times the nominal thickness of the coating layer. The characteristiclength L may be 100 μm or less, such as 75 μm or less, 50 μm or less, 25μm or less, 10 μm or less, 1 μm or less, 500 nm or less, or 200 nm orless.

FIG. 4(a) is a schematic top view of FIG. 3(a), illustrating the shapeof the ablated edge 30 on a microscale. The edge 30 is characterized bya scalloped shape, resulting from partially overlapping laser spotlocations with a round or circular laser beam cross-section. As shown,the edge 30 is not perfectly straight when viewed on a size scale closeto that of the laser spot size. The shape of the illustrated edge 30 maybe uniform, however, even if not perfectly straight or smooth. Forinstance, when the laser ablation process is configured so that thelaser beam and workpiece move at a constant relative speed with laserpulses delivered to the workpiece at a constant frequency, the processededge 30 has a periodic shape with equal peak-to-peak andvalley-to-valley spacing D in the process direction, as shown. Thepeak-to-valley distance d, measured in a direction transverse to theprocess direction, may increase with increased process motion or speedand decrease with increased laser pulse frequency, both of which arerelated to the amount of overlap associated with adjacent laser spotlocations. In one example, where the distance D between laser spotlocations is about one-third the diameter of the laser beam, the depth dof the scallops may be about 2-5% the diameter of the laser beam. Asmaller distance D, such as one-quarter the diameter of the laser beam,leads to a smaller depth d, such as about 2% of the diameter of thelaser beam. A larger distance D, such as one-half the diameter of thelaser beam, leads to a larger depth d, such as about 6-8% of thediameter of the laser beam. The depth d may be about 1-20% of thediameter of the laser beam, such as 2-8% of the diameter of the laserbeam. Stated quantitatively, the depth d may be 100 μm or less, such as75 μm or less, 50 μm or less, 25 μm or less, 10 μm or less, 1 μm orless, 500 nm or less, or 200 nm or less.

While the edge 30 may not be perfectly smooth or linear on theillustrated microscale, the periodic uniformity of the processed edgehelps provide a smooth appearance when perceived by the naked eye. Asshown in FIG. 4(b), a material addition process with masking producededge 30′ is also not perfectly smooth on a microscale. Thenon-uniformity along the edge 30′ shown in FIG. 4(b) leads to a macrovisual appearance that may be perceived as an unsmooth edge, even if theaverage peak-to-valley depth d is the same as that produced with thelaser ablation process.

FIG. 5 illustrates an embodiment in which the laser ablation process isperformed along a perimeter of the intended ablation area 26 before theremaining portion within the perimeter is ablated. The speed of thelaser beam with respect to the workpiece 10 along the process path maybe constant in the instantaneous direction of movement along theperimeter of the intended ablation area 26 to achieve a uniform edge atthe perimeter. Performing the ablation process such that a portion ofthe overall process path follows the shape of the perimeter of theintended ablation area 26 facilitates use of a larger laser beamcross-section and shorter process times by enabling the use of lessoverlap in one of the index axes while also providing a processed edgeat the perimeter of the ablation area that has a smooth appearance,particularly with non-rectangular ablation areas that have curvilinearedges such as in FIGS. 2 and 5. Alternatively, the perimeter of theablation area can be the final portion of the area from which thecoating layer is removed.

The process is of course not limited to removing the entire coatinglayer in any particular area of the workpiece. The laser ablationprocess can be used to selectively remove coating material to formdecorative patterns, functional patterns, and/or indicia, for example.Desired patterns or indicia can be formed from the portion of thecoating layer remaining over the substrate after the ablation process,or they can be formed by the ablated area itself. Second surfaceablation has the additional advantage that, due to the at leastpartially transparent substrate, decorative features or indicia can beviewed through the first side of the finished product. The ablatedworkpiece can be assembled with the remaining coating layer facingtoward the inside of an assembly such that it is protected from damageand from the environment by the substrate.

FIG. 6 illustrates one example of a component 32 that can be formed fromthe ablated workpiece. Component 32 is taken from the interior of theworkpiece 10 of FIG. 5 by cutting, scoring, or otherwise separating itfrom the surrounding portion of the ablated workpiece. In oneembodiment, a series of laser induced channels can be formed in thesubstrate along a desired line of separation to facilitate removal ofthe component 32 from the ablated workpiece. Examples of laser inducedchannels and processes for forming them in a substrate are described ingreater detail by Bareman et al. in U.S. Pat. No. 8,842,358. An edge 34of the component 32 is formed along the line of separation. In thisexample, the edge 34 circumscribes the ablated area 24 formed during theablation process and is generally parallel with the processed edge 30 ofthe remaining coating layer. The component 32 thus formed includes awindow 36 with the transparency of the substrate and a border 38 havingthe optical and other physical properties of the coating layer material.

The border 38, and in fact the coating layer of the original workpiece,may be formed from nearly any material (e.g., metallic, plastic and/orceramic) and may generally be less transparent than the substrate.Certain metallic materials, such as chromium or chromium-containingmaterials, may be multi-functional, providing reflectivity, opacity,conductivity, along with a potentially decorative aspect. In someembodiments, the coating layer as provided to the ablation process isitself a multi-layer coating. For instance, the coating layer mayinclude a reflective layer in direct contact with the substrate and alight-absorbing layer over the reflective layer to minimize reflectionof the laser light in the ablation process.

In one embodiment, the component 32 or similar component having acoating layer from which material has been laser ablated, is a mirrorcomponent, such as a component of a vehicle rearview mirror assembly.The border 38 of the component 32 may serve to eliminate the need for aseparate frame for such a mirror and may also serve other functions,such as providing electrical conductivity, electrical insulation,reflectivity, and/or concealing electrical connections or other mirrorassembly components. In one particular example, the component 32 is thefront piece of an electrochromic mirror assembly in which anelectrochromic medium is encapsulated in a cavity formed between theback side of the component 32 (i.e., the second side 16 of the originalworkpiece 10 of FIG. 1) and a second similarly shaped component. Someexamples of electrochromic mirror assemblies are also given in theabove-referenced U.S. Pat. No. 8,842,358 and in some of the documentsreferenced therein. Other non-mirror electrochromic devices (e.g.,electrochromic windows or lenses) may also be formed from the ablatedworkpiece, as can non-electrochromic assemblies.

Some devices that may employ at least a portion of the laser ablatedworkpiece, such as electrochromic devices, may require one or moreelectrically conductive layers such as an electrode layer. In anelectrochromic device, for example, electrodes may be included onopposite sides of the electrochromic medium wherever it is desired toactivate the electrochromic medium in the device. The component 32 maythus also include an electrically conductive layer along at least aportion of the window 36, corresponding to the ablated portion 24 of theoriginal workpiece. The electrically conductive layer may be formed froma transparent conductive oxide (TCO) or other suitable conductivematerial, such as indium tin oxide (ITO). In one embodiment, theconductive layer overlies the entire window 36.

As shown in FIG. 7, the above-described second surface laser ablationprocess is compatible with TCO materials or other at least partiallytransparent conductive layers. The workpiece 10 in the illustratedprocess includes an electrically conductive layer 40 at the second side16 of the workpiece between the substrate 12 and the coating layer 14.The conductive layer 40 provides the second surface 20 from which thecoating layer 14 is removed, in this example. The illustrated processrepresents an example of a laser ablation process in which the laserbeam 100 propagates through the conductive layer, such as a metalliclayer, to remove material from an opposite side of the conductive layer.In other embodiments, the electrically conductive layer may be disposedover the second side of the workpiece after the ablation process. Thisalternative allows for application of the conductive layer only onselected workpieces. The laser wavelength may be selected to minimizeabsorption by the conductive layer. In one non-limiting example, a laserhaving a wavelength of 532 nm is used with an ITO conductive layer inorder to minimize damage to the conductive layer.

While the above-described laser ablation process can provide a processedworkpiece with coated and uncoated portions without the need for themasking that is typical of material addition coating processes, maskingcan be advantageously employed in the laser ablation process. Maskedlaser ablation can form features that are sharper than those formed inmasked material addition coating processes and, in some cases, sharperthan those formed by laser ablation alone. For instance, when a desiredcharacteristic feature size is smaller than the cross-sectional size ofthe laser beam, masked ablation can be used to obtain such featureswithout the negative effects associated with masked material additioncoating processes.

One example of a masked laser ablation process is illustrated in FIG. 8.In this example, a mask 42 is provided at the first side 18 of theworkpiece. The mask 42 includes open or otherwise light-transmittingportions and solid or otherwise light-filtering portions. When the laserbeam 100 encounters the solid portions of the mask 42 while movingrelative to the workpiece 10 in the process direction A, the beam isselectively blocked by the solid portions of the mask. Features 44, suchas indicia, are thereby formed directly opposite the solid portions ofthe mask 42 in the form of unablated portions of the coating layer. Thesolid portions of the mask 42 need not be completely opaque orlight-blocking. It is only necessary to attenuate the laser beam by anamount sufficient to prevent ablation at the second surface 20. In fact,the masked ablation process facilitates optimization of certain aspectsof the ablation process such that preventing only a small portion of thelight from being transmitted through the substrate may be necessary toform features 44.

For instance, one manner of optimizing the laser ablation process is tomaximize the removal rate of the coating layer 14 by maximizing thecross-sectional size of the laser beam 100 and the associated laser spot104 (e.g., via selection of laser optics), along with the speed at whichthe laser is rastered along the workpiece 10. This optimization islimited by the flux at the second surface 20 being reduced as the squareof the beam radius at the surface. Above a threshold spot size, theenergy flux falls below the ablation threshold for the coating layer,resulting in a net loss of performance. It is thus useful to configurethe laser spot size and raster speed to just above the ablationthreshold to reduce the process cycle time. A large spot size improvesoverall coating removal rate, but it may limit the size scale on whichindicia can be formed, in the absence of masking. For example, if a 200micron diameter laser spot size is used to rapidly remove the coatinglayer, smooth and/or fine features on a 50 or 100 micron scale cannot beformed, whether part of indicia or other features, due both to theoverall size of the spot and its round shape. Employing a non-circularbeam (e.g., rectangular) can help eliminate the above-describedscalloped shape of the processed edge and reduced the amount of overlaprequired by adjacent laser spot locations. But formation of featuressmaller than the laser spot is problematic, even with shaped beams. Someprocesses employ a second, smaller beam to form the small features whileusing a larger optimized beam to remove the bulk of the coating layermaterial.

The masked laser ablation process eliminates the need for a second laserbeam and the associated second process path, resulting in a much fasterprocess that can use a single optimized beam. As used in the laserablation process, the mask 42 provides other process advantages. Forinstance, there is no coating material deposited on the mask 42 in thelaser ablation process as is sometimes the case in masked coatingprocesses. Also, the mask may be formed from materials that are notcompatible with masked coating processes. For example, some coatingprocesses are performed at high temperatures and/or with chemicallyactive or reactive materials. The masks used in such processes mustwithstand these harsh conditions, while the mask 42 used in the laserablation process is not exposed to high temperatures or a reactiveenvironment. The mask 42 only encounters the laser beam 100. Moreover,the mask 42 is located away from the focal plane of the laser beam 100and is thus less affected or relatively unaffected by the energy of thebeam.

In one embodiment, the mask 42 is formed in place on the first side 18of the workpiece 10. Photolithography is one process that can form themask 42 in the desired pattern. But photolithography can be expensive,time-consuming, and may require a mask of its own. Another method offorming the mask 42 in place on the first side 18 of the workpiece is byprinting. A printing technique such as inkjet printing can be used toform the mask 42 by selectively depositing the mask material along theworkpiece 10. With the mask 42 formed in place by printing, the sizescale of the features 44 that can be formed by laser ablation is limitedby the resolution of the printing technique rather than by the laserspot size. Alternatively, a pre-formed mask may be placed over the firstside 18 of the workpiece. The pre-formed mask may be held in place overthe workpiece by any appropriate means, such as a clamp. The pre-formedmask may be reusable, such that it may be employed in the laser ablationof multiple workpieces.

In some cases, the above-described laser ablation process results in anablated area with a measurable transmission haze. Transmission haze mayresult from diffusion or scattering of some of the light passing throughthe ablated substrate. While the exact cause of the haze is not fullyunderstood, it may be attributed to residual coating layer materialand/or inter-compounds of the coating material and the underlyingmaterial. The haze may also be partly attributed to some roughening,damage, or other material change at the removal surface, whether causeddirectly by the energy of the laser beam, or indirectly by forces orother phenomena resulting from the separation of the coating layer fromthe underlying material. The ablated area of the workpiece may have atransmission haze of 0.25% or less, on average, such as 0.2% or less,0.15% or less, or 0.1% or less. It is possible to form the ablated areaof the workpiece with a transmission haze of 0.05% or less, on average.In some cases, the haze may be higher, and the maximum allowable hazemay depend on the intended use of the workpiece.

One manner of reducing the haze associated with the laser ablationprocess is through the use of a laser system that delivers pulses of thelaser beam on a picosecond or shorter time scale. Picosecond lasers areconfigured to deliver the energy necessary for coating material removalin laser pulses with durations in a range from about 0.5 to about 500picosends (ps). Pulse durations of several tens of picoseconds may bepreferred, such as 1-50 ps or 50 ps or less. Commercially availablepicosecond lasers can provide pulse durations of less than 20 ps, lessthan 10 ps, less than 5 ps, or less than 1 ps, to name a few.Femtosecond lasers having a pulse duration in a range from about 0.5 toabout 500 femtoseconds (fs) can provide some of the same advantages aspicosecond lasers when compared with nanosecond lasers (0.5 to 500 nspulse duration).

FIGS. 9(a) and 9(b) are photomicrographs of processed edges of metalliccoating layers 14 formed by laser ablation. A picosecond laser was usedto remove the coating layer 14 from the substrate 12 in FIG. 9(a), and ananosecond laser was used to remove the coating layer from the substratein FIG. 9(b). The amount of haze at the ablated area of the picosecondlaser ablated workpiece is visibly less than that of the nanosecondlaser ablated workpiece. Generally, the heat-affected zone is smallerwith the picosecond laser. In addition to the lower amount of hazeassociated with the picosecond laser, the uniform (i.e., periodic) shapeof the processed edge is more apparent than with the nanosecond laser,possibly due to less molten coating material being formed duringprocessing. The nanosecond laser processed edge of FIG. 9(b) also hasapparent microcracks extending away from the edge and into the remainingcoating layer. The microcracks are about 10-15 microns long, on average,and are spaced along the processed edge about every 5-15 microns.

Another aspect of the invention is a method comprising the step ofdirecting a laser beam at a first side of a workpiece to remove materialfrom an opposite second side of the workpiece. The workpiece maycomprise a substrate that is at least partially transparent to the laserbeam and a coating layer comprising the removed material. The workpiecemay also comprise an electrically conductive layer that is not removedby the laser beam. The method may also comprise the step of providing amask to selectively prevent material removal from the second side of theworkpiece. The method may also comprise the step of providing a printedmask at the first side of the workpiece. The method may also comprisethe step of forming a laser ablated feature with a characteristicdimension that is smaller than a laser spot defined by the laser beam.The method may produce a processed edge of a coating layer from whichmaterial is removed by the laser beam that tapers from a nominal coatinglayer thickness to zero thickness over a distance that is less thantwice the nominal thickness. The method may produce a processed edge ofa coating layer from which material is removed by the laser beam thatforms an average angle with an underlying substrate surface in a rangefrom 30 to 90 degrees. The method may produce a processed edge of acoating layer from which material is removed by the laser beam that hasa uniform shape. The method may also comprise the steps of removingmaterial along a perimeter of an intended ablation area before removingother material within the intended ablation area. The laser beam may beprovided by a picosecond pulsed laser. The removed material may bemetallic and reflective when part of the workpiece.

Another aspect of the invention is a product comprising at least aportion of a workpiece produced according to the method described above.The product may comprise a component formed from the workpiece byseparating the workpiece into more than one portion. The product maycomprise a vehicle rearview mirror assembly. The product may alsocomprise an electrochromic medium located between electrode layers, withat least one of said electrode layers being located on the workpiece.The product may also comprise an electrically conductive layer thatprovides the second surface. The laser-processed edge of the product maynot be a scalloped edge. The product may also comprise a laser ablatedfeature with a characteristic dimension that is smaller than a laserspot defined by the ablating laser beam. The processed edge may taperfrom a nominal coating layer thickness to zero thickness over a distancethat is less than twice the nominal thickness. The processed edge mayform an average angle with an underlying substrate surface in a rangefrom 30 to 120 degrees. The processed edge may have a uniform shape. Thecoating layer material may be metallic and reflective. The product mayalso comprise an electrochromic medium located between electrode layers,with at least one of said electrode layers being located along thelaser-ablated portion of the substrate.

Another aspect of the invention is a product comprising a substrate thatis at least partially transparent and a coating layer over a portion ofthe substrate, the coating layer having a laser-processed edge definedbetween the coated portion and a second surface laser-ablated portion ofthe substrate. The coating layer of the product is less transparent thanthe substrate material. The product may also comprise an electricallyconductive layer that provides the second surface. The laser-processededge of the product may not be a scalloped edge. The product may alsocomprise a laser ablated feature with a characteristic dimension that issmaller than a laser spot defined by the ablating laser beam. Theprocessed edge may taper from a nominal coating layer thickness to zerothickness over a distance that is less than twice the nominal thickness.The processed edge may form an average angle with an underlyingsubstrate surface in a range from 30 to 120 degrees. The processed edgemay have a uniform shape. The coating layer material may be metallic andreflective. The product may also comprise an electrochromic mediumlocated between electrode layers, with at least one of said electrodelayers being located along the laser-ablated portion of the substrate.

Another aspect of the invention is a vehicle rearview mirror assemblyincluding the product described above.

While certain embodiments have been illustrated and described, it shouldbe understood that changes and modifications can be made therein inaccordance with ordinary skill in the art without departing from thetechnology in its broader aspects as defined in the following claims.

The embodiments, illustratively described herein may suitably bepracticed in the absence of any element or elements, limitation orlimitations, not specifically disclosed herein. Thus, for example, theterms “comprising,” “including,” “having,” “containing,” etc. shall beread expansively and without limitation. Additionally, the terms andexpressions employed herein have been used as terms of description andnot of limitation, and there is no intention in the use of such termsand expressions of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the claimed technology.Additionally, the phrase “consisting essentially of” will be understoodto include those elements specifically recited and those additionalelements that do not materially affect the basic and novelcharacteristics of the claimed technology. The phrase “consisting of”excludes any element not specified.

The present disclosure is not to be limited in terms of the particularembodiments described in this application. Many modifications andvariations can be made without departing from its spirit and scope, aswill be apparent to those skilled in the art. Functionally equivalentmethods and compositions within the scope of the disclosure, in additionto those enumerated herein, will be apparent to those skilled in the artfrom the foregoing descriptions. Such modifications and variations areintended to fall within the scope of the appended claims. The presentdisclosure is to be limited only by the terms of the appended claims,along with the full scope of equivalents to which such claims areentitled. It is to be understood that this disclosure is not limited toparticular methods, reagents, compounds compositions or biologicalsystems, which can of course vary. It is also to be understood that theterminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting.

In addition, where features or aspects of the disclosure are describedin terms of Markush groups, those skilled in the art will recognize thatthe disclosure is also thereby described in terms of any individualmember or subgroup of members of the Markush group.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the like,include the number recited and refer to ranges which can be subsequentlybroken down into subranges as discussed above. Finally, as will beunderstood by one skilled in the art, a range includes each individualmember.

All publications, patent applications, issued patents, and otherdocuments referred to in this specification are herein incorporated byreference as if each individual publication, patent application, issuedpatent, or other document was specifically and individually indicated tobe incorporated by reference in its entirety. Definitions that arecontained in text incorporated by reference are excluded to the extentthat they contradict definitions in this disclosure.

Other embodiments are set forth in the following claims.

What is claimed is:
 1. A method comprising: providing a workpiececomprising a substrate, a coating layer disposed over a first surface ofthe substrate, and a mask disposed over a second surface of thesubstrate; and removing a portion of the coating layer from theworkpiece by directing a laser beam at the workpiece such that the laserbeam passes through the substrate from the second surface to the firstsurface before the laser beam impinges on the coating layer; wherein thefirst surface of the substrate and the second surface of the substrateare opposite surfaces of the substrate, and the mask selectivelyprevents removal of the coating layer from portions of the workpiece. 2.The method of claim 1, further comprising disposing the mask over thesecond surface of the substrate.
 3. The method of claim 2, whereindisposing the mask comprises a photolithography process or a printingprocess.
 4. The method of claim 2, wherein disposing the mask comprisesplacing a pre-formed mask over the second surface of the substrate. 5.The method of claim 1, wherein removing the portion of the coating layerproduces portions of the workpiece from which the coating layer has beenremoved that have a characteristic dimension that is smaller than alaser spot produced by the laser beam.
 6. The method of claim 1, whereinan edge of the coating layer adjacent the portion of the workpiece fromwhich the coating layer has been removed has a characteristic length, L,that is a length of the portion of the edge of the coating layer inwhich a thickness of the coating layer tapers from a nominal coatinglayer thickness, t, to zero thickness, and L≦100 μm.
 7. The method ofclaim 6, wherein L≦50 μm.
 8. The method of claim 7, wherein L≦200 nm. 9.The method of claim 1, wherein an edge of the coating layer adjacent theportion of the workpiece from which the coating layer has been removedhas a uniform shape.
 10. The method of claim 1, wherein an edge of thecoating layer adjacent the portion of the workpiece from which thecoating layer has been removed has a scalloped profile.
 11. The methodof claim 10, wherein a depth of the scallops is between 1% and 20% of adiameter of the laser beam.
 12. The method of claim 10, wherein a depthof the scallops is less than 100 μm.
 13. The method of claim 1, whereinthe laser beam is produced by a laser with a pulse duration in a rangeof 0.5 to 500 picoseconds.
 14. The method of claim 1, wherein theworkpiece further comprises an electrically conductive layer disposedbetween the substrate and the coating layer, and the electricallyconductive layer is not removed by the laser beam.
 15. The method ofclaim 14, wherein the electrically conductive layer comprises atransparent conductive oxide.
 16. The method of claim 15, wherein thetransparent conductive oxide comprises indium tin oxide.
 17. The methodof claim 1, wherein the coating layer comprises a metallic material. 18.The method of claim 17, wherein the metallic material compriseschromium.
 19. The method of claim 1, wherein the coating layer comprisesmultiple layers.
 20. The method of claim 1, wherein an edge of thecoating layer adjacent the portion of the workpiece from which thecoating layer has been removed forms an average angle with the firstsubstrate surface in a range of 30° to 90°.
 21. The method of claim 1,wherein the portion of the workpiece from which the coating layer isremoved exhibits a transmission haze of 0.05% or less.
 22. The method ofclaim 1, wherein the portion of the workpiece from which the coatinglayer is removed exhibits a transmission haze of 0.25% or less.
 23. Amethod comprising: providing a workpiece comprising a substrate and acoating layer disposed over a first surface of the substrate; andremoving a portion of the coating layer from the workpiece by directinga laser beam at the workpiece such that the laser beam passes throughthe substrate before the laser beam impinges on the coating layer;wherein the laser beam is produced by a laser with a pulse duration in arange of 0.5 to 500 femtoseconds.
 24. The method of claim 23, whereinthe workpiece further comprises a mask disposed over a second surface ofthe substrate, the first surface of the substrate and the second surfaceof the substrate are opposite surfaces of the substrate, and the maskselectively prevents removal of the coating layer from portions of theworkpiece.
 25. A product comprising: a substrate; a coating layerdisposed over a first surface of the substrate; and a portion of thefirst surface of the substrate that is substantially free of the coatinglayer; wherein an edge of the coating layer adjacent the portion of thefirst surface of the substrate that is substantially free of the coatinglayer forms an average angle with the first substrate surface in a rangeof 30° to 120°, the edge of the coating layer adjacent the portion ofthe first surface of the substrate that is substantially free of thecoating layer has a characteristic length, L, that is a length of theportion of the edge of the coating layer in which a thickness of thecoating layer tapers from a nominal coating layer thickness, t, to zerothickness, and L≦100 μm.
 26. The product of claim 25, wherein L≦50 μm.27. The product of claim 26, wherein L≦200 nm.
 28. The product of claim25, further comprising an electrically conductive layer disposed betweenthe substrate and the coating layer and over the portion of the firstsurface of the substrate that is substantially free of the coatinglayer.
 29. The product of claim 25, wherein the coating layer comprisesa metallic material.
 30. The product of claim 25, wherein the portion ofthe first surface of the substrate that is free of the coating layerexhibits a transmission haze of 0.05% or less.
 31. The product of claim25, wherein the portion of the first surface of the substrate that isfree of the coating layer exhibits a transmission haze of 0.25% or less.32. The product of claim 25, wherein the edge of the coating layeradjacent the portion of the first surface of the substrate that issubstantially free of the coating layer has a scalloped profile.
 33. Themethod of claim 32, wherein a depth of the scallops is less than 100 μm.34. The method of claim 33, wherein a depth of the scallops is less than50 μm.
 35. The product of claim 25, further comprising: a firstelectrode layer; a second electrode layer; and an electrochromic layer;wherein the electrochromic layer is disposed between the first electrodelayer and the second electrode layer, and the first electrode layer isdisposed over the portion of the first surface of the substrate that isfree of the coating layer.
 36. A vehicle rearview mirror assemblycomprising a product comprising: a substrate; a coating layer disposedover a first surface of the substrate; and a portion of the firstsurface of the substrate that is substantially free of the coatinglayer; wherein an edge of the coating layer adjacent the portion of thefirst surface of the substrate that is substantially free of the coatinglayer forms an average angle with the first substrate surface in a rangeof 30° to 120°, the edge of the coating layer adjacent the portion ofthe first surface of the substrate that is substantially free of thecoating layer has a characteristic length, L, that is a length of theportion of the edge of the coating layer in which a thickness of thecoating layer tapers from a nominal coating layer thickness, t, to zerothickness, and L≦100 μm.